1. Field of the Invention
The present invention relates in general to a MOS differential input stage, and more particularly to a high voltage MOS differential input stage incorporating input gate protection.
2. Description of the Related Art
Years ago, 15 Volt (V) MOS field-effect transistors (MOSFETs) had thick gate oxides in which the breakdown voltage of the gate oxide exceeded the drain-source breakdown voltage. There was not a problem with rupturing the gate oxide of amplifiers or comparators having input stages that used these earlier MOSFET configurations. As oxides got thinner, the breakdown voltage got smaller. Eventually, DMOS (double-diffused MOS) transistors came out that had higher voltages. But there was a problem inasmuch as the operating voltage of the FETs, such as, for example, 40V, was far greater than the rated breakdown voltage of the gate oxides for these devices.
One solution to this problem was to add back-to-back external diodes across the input pair. This worked for some amplifier applications but not for comparators in which one input might be connected to a voltage reference while the other input might be at any voltage. Conventional methods have been proposed for solving this problem with internal designs in which the input transistor devices are operated as source followers. Then the voltages at their sources became a new input signal pair for an internal amplifier that was configured to handle large input voltage differences. The conventional methods have been complicated in that they require significant design effort for each implementation.
JFET (junction FET) amplifiers have also been available. Although JFETs do not have a gate oxide breakdown problem, they do incorporate fairly large diodes that leak current, especially at high temperatures, and are thus undesirable for many applications.